In precision systems that perform operations on workpieces and the like, the workpiece is placed on, held by, and moved as required by a stage or other device that undergoes controlled motion relative to a tool, optical system, energy source, or other implement that performs the operation(s) on the workpiece. Such motion can be achieved by any of various actuators, but linear motors have become favored due to their wide range of motion, accuracy, precision, reliability, and simplicity. A linear motor is an electromagnetic actuator in which the stator extends in a substantially one-dimensional (linear) manner and in which the mover moves relative to the stator in either direction within the one dimension. In a moving-coil linear motor, the stator is a linear array of permanent magnets and the mover comprises a linear array of wire coils that, when electrically actuated, moves relative to the magnet array.
A planar motor is similar to a linear motor in many aspects, but the stator of a planar motor extends in a substantially two-dimensional (planar) manner and the mover moves relative to the stator in substantially any direction within the two dimensions. In a moving-coil planar motor, the stator is a planar array of permanent magnets and the mover comprises a two-dimensional array of wire coils. Planar motors, as well as linear motors, operate very accurately and precisely, especially in conjunction with displacement- and position-measuring devices such as encoders and interferometers.
Modern microlithography systems have at least one movable stage (e.g., a “reticle stage” or “wafer stage” or “substrate stage”). For actuations of these stages, linear motors are widely used. The substrate stage or wafer stage is often substantially larger than the reticle stage, and in certain microlithography systems is quite massive. Especially for these applications, planar motors are being favorably considered.
Initialization of the wafer stage of a microlithography system must be performed each time the system is restarted. During a conventional restart, the location and orientation of the wafer stage are unknown to the system. Since the primary metrology system for a wafer stage usually involves relative measurements (i.e., interferometry), the initialization process requires movement of the stage to and measurement of a known absolute position to establish a reference origin. Typically, a conventional initialization includes moving the stage to a set of initialization sensors that can determine an initial absolute orientation and position of the stage, but have a very limited measurement range. Movement of the stage to the initialization sensors can often be performed manually, but this manual initialization procedure is cumbersome and time-consuming.
Stages for conventional lithography systems typically utilize air bearings to support the stage and stacked linear motors to provide large-amplitude motions of the stage in the two main movement directions (x and y). Whereas a stage driven by linear motors can be equipped with absolute encoders along the motor axes to provide data on approximate commutation positions, this procedure is impractical for use with a stage driven by a planar motor.
Recently, substantial development effort has been directed to use of planar motors for producing stage movement at least in the x- and y-directions. A planar motor is similar in certain ways to a linear motor; but, a planar motor provides significant motion in two dimensions (x and y; “2-D”) rather than in one dimension as achieved by a linear motor. The motions produced by a moving-coil planar motor are relative to an x-y (planar; 2-D) array of magnetic fields (nowadays produced by a corresponding array of permanent magnets). The magnet array constitutes the stator and hence is part of the planar motor.
The movable portion of a moving-coil planar motor (together with any mass being carried by the movable portion) is typically magnetically levitated relative to the planar array of magnets. Commutation requires “knowledge” of the stage's position, relative to the magnetic-field array, in both the x- and y-directions. This data must be obtained before the movable portion of the planar motor can be levitated or moved at all. In contrast to linear motors, with a maglev planar motor no mechanical guide is used to control yaw (θz motions) during movements of the stage. Hence, a yaw measurement is also required (in addition to the x- and y-position measurements) to initiate stable levitation and movement of the stage.
Accurate measurement of stage position involves measurements in all six possible degrees of freedom, namely x, y, z, θx, θy, θz. If the stage is being moved in the y-direction, θx denotes pitch and θy denotes roll associated with the y-direction main motion. For one conventional stage system, z-position, θx, and θy are measured using capacitative sensors attached to the stage itself. Since these sensors move with the stage, they are always available to provide respective their respective data regardless of the stage position. But, these sensors do not provide x-position, y-position, or θz data for initialization. (Note that θx, θy, and θz are also termed Tx, Ty, and Tz, or theta-x, theta-y, and theta-z, respectively.) For measuring x-position, y-position, and θz for initialization, other sensors (“initialization sensors”) have been tried; but, since the initialization sensors are fixed at the initialization position, they can be used only after the stage has been moved to the initialization position.
It is impractical in a production microlithography machine to include a device for (or manually) moving the stage to an initialization position each time initialization is required. Eliminating such a device does not eliminate the need to obtain initial x-position, y-position, and θz data for the stage.